I always say "I couldn't care less," because double negatives tickle me. I can't escape the feeling that I'm being a tad haughty when I do, because where I live people don't use double negatives to actaully mean a positive.

The misuse of homonyms drive me insane! There, their, they're are always used incorrectly! To, too, two. And don't even get me started on pronunciation problems... "can I aks you a question?" "No you cannot aks me a question, you can ASK me a question." It drives me up the wall.